1. Field of the Invention
The present invention relates to an X-ray apparatus of the type having an X-ray examination system which with an X-radiation source and an X-ray detector which can be displaced relative to an examination subject for the pickup of 2D projections of a region of the subject, with subsequent reconstruction of 3D images of the region of the subject.
2. Description of the Prior Art
X-ray apparatuses of the above type commonly have a C-arm for mounting the X-ray source and the X-ray detector, the C-arm being mounted in a holding device such that it can be displaced in motorized fashion along its perimeter in a defined angle range (orbital motion). To obtain 2D projections from various projection angles for the reconstruction of 3D images--of a body region of a living organism, for example--in the pickup of the 2D projections of the body region of the organism, the C-arm is displaced along its perimeter subsequent to corresponding placement relative to the living organism to be examined. 3D images of the body region of the organism are subsequently reconstructed from the 2D projections captured with the X-ray examination system during the displacing motion. The reconstruction of 3D images is preconditioned by the precise knowledge of the projection geometries, i.e. the knowledge of the positions and orientations of the X-ray source and of the X-ray detector with respect to a stationary coordinate system during each of the individual 2D projections.
It has proven problematic that known stationary C-arm X-ray apparatuses, and quite particularly mobile C-arm X-ray devices, exhibit mechanical instabilities, particularly with respect to the displacement of the C-arm along its perimeter, so that the actual displacing motion of the X-ray examination system deviates from the ideal displacing motion due to deformations of the C-arm. Thus, the precision in the reproducibility of the projection geometries which is necessary for a reconstruction of 3D images cannot be achieved, particularly with the known mobile C-arm X-ray devices, for which reason additional position detection systems are necessary in order to be able to determine the projection geometries in every 2D projection. The following two methods are known for determining the projection geometries:
a) German OS 195 12 819 (corresponding to U.S. Pat. No. 5,706,324) teaches the utilization of a marker ring, usually made of plexiglass with inserted metal structures, which is arranged around the body region of the examined organism. The metal structures of the marker ring are visible in the 2D projections of the examined body region, so that the respective projection geometries of the 2D projections can be calculated from their position. This method has the disadvantage that the marker ring has a relatively large diameter, so that the distance between the X-ray source and the marker ring is very small (a few centimeters), particularly given mobile C-arm X-ray devices having a relatively small C-arm. The metal structures are thus imaged with significant enlargement in the 2D projections, so that large parts of the 2D projections are covered by the metal structures. Furthermore, only a small region of the metal structures of the marker ring is imaged in the 2D projections, so that the determination of the projection angle with the aid of the low number of imaged metal structures is difficult. PA1 b) Gauging measurements are performed prior to the actual patient measurement, under the assumption that the system behavior, i.e. essentially the displacement of the C-arm, is largely reproducible. This method is very time-consuming and can be used only given mechanically reinforced stationary C-arm X-ray devices. Application in mobile X-ray devices is impossible, due to the previously mentioned mechanical instabilities of such X-ray devices, mechanical stabilzation being out of the question for mobile X-ray devices due to the large weight increase, which restricts mobility.